Silt fence, or filter fabric fence, is a temporary linear sediment barrier of permeable fabric designed to intercept and slow the flow of sediment-laden sheet flow runoff from areas of erodible soil. Silt fence is made of woven synthetic filtration fabric (also known as geosynthetic or geotextile). Geotextiles are manufactured by tightly stretched lengthwise polymer strands, known as warp, woven with filler polymer strands, known as weft. These materials often contain identical warp and weft strands creating what is known as an even weave. The conventional silt fence has uniform geotextile weaving producing a constant permittivity (permittivity=cross-plane permeability coefficient / thickness at specified normal pressure) throughout the material's height and length. Mud cake reduction in permittivity is also uniform for conventional silt fence. Typical width of the geotextile is 24 to 36 inches. Any desired length of the geotextile can be manufactured as it is transported in rolls.
Proper installation of the woven geotextile requires support vertically by steel or wood posts in plurality, properly spaced to such length as to be substantially strong to hold the geotextile silt fence upright while impounding water to its capacity. The silt fence material should be stretched between support posts to assure it is taunt to prevent sag failure. Said fences require installation in a linear fashion along a constant topography and entrenched to create a seal with the earth. Because said fence is entrenched throughout its length into the soil, it creates a vertical hydraulic barrier providing a temporary impoundment. Therefore, the entrenched portion must be water tight to provide a vertical slurry barrier with the soil. Therefore, larger storms, either by long duration or high intensity, create greater depth of impounded water behind the fence.
The purpose of silt fence is to prevent sediment carried by sheet flow stormwater runoff from leaving denuded areas such as construction sites and entering natural waters or storm drainage systems. This best management practice treats sediment-laden stormwater runoff and reduces erosion by slowing the velocity of runoff, thereby causing the deposition of sediment at the structure interface. Silt fencing encourages sheet flow at the discharge as a level spreader and reduces the potential for development of rills and gullies which are aggressive forms of erosion.
Silt fence is widely utilized as a best management practice for construction activities which denude soil, exposing it to erosion. Because of their widespread use, systematic failure of silt fence devices allows significant sediment loads to reach natural waters. Typically, failure of silt fence occurs from two mechanisms: improper installation or hydraulic overtopping.
Improper installation leads to such failures as sediment-laden water flowing around or underneath the silt fence. When silt fence is impounding runoff at a depth equivalent to the installed fence height, vertical portions of the hydrostatic forces at the wetted fence interface create a concave structure perpendicular to the horizontal ground surface centered nearly at the center of the height of the silt fence.
Overtopping failure occurs because excessive stormwater runoff impounded by the silt fence forces the material to sag by physical submersion and subsequent loss of the uplifting vertical hydrostatic forces acting upon the upper portions of the installed fence. Loss of the upward containment forces on the geotextile allows reduction in height of the fence in upper portions of the fence. The loss of uplift forces in this critical location creates catastrophic failure of the device. A significant amount of water and suspended sediment impounded by the fence is released quickly in these sag failures. Because the failure discharge is uncontrolled, relatively high discharge velocities are common. High velocities transport significant loads of sediment to natural waters. Elimination of sag failure by control of the overflow would reduce discharge velocities minimizing sediment load discharged.
Overtopping occurs frequently because silt fence has not previously been designed to provide higher flow rate for greater intensity and duration storm events. If the fence could allow greater flow rates at higher levels in the barrier, the frequency of catastrophic overtopping failures would be significantly reduced. In addition, if the barrier had a non-overtopping location intended for allowance of overflow, elimination of overtopping failures would be achieved. These two overtopping failure elimination devices are a product of this invention.